1.3 Impedance track benefits

This is David Maxwell back again for Part 3 of Single-Cell Gauging 101. Previously we talked about battery basics and then we talked about the classic gauging methods for lithium ion batteries, namely voltage methods and coulomb counting methods. We talked about some of the problems that those have, and now we're going to talk about how Impedance Track solves those problems and some of the benefits that it gives you.
Voltage-based gauges were fine under no load and coulomb counting gauges were good when there was a load, although even that still had problems. Impedance Track combines the advantages of both of these and then adds the feature of real-time impedance measurement, which is where it gets its name, impedance tracking. It's tracking it over time, over cell-to-cell variation, and most importantly, as the battery ages, since that will happen at a different rate for every battery.
Impedance Track algorithm is basically a predictive algorithm that will calculate the remaining run time under any load that you want. And it's going to combine the open circuit voltage measurements with the impedance information as well as the coulomb counting to predict how much capacity you have left under any temperature, under any load, and will be able to maintain its accuracy even as the battery ages. So that's how Impedance Track in a nutshell solves the problem of this variable resistance which we saw cause so many problems with the classic gauging methods.
Let's quickly look at the open circuit voltage profile of several different manufacturers' batteries, standard, lithium cobalt dioxide, chemistry, the 4.2 max charging voltage. If you overlay them, you can see they're usually pretty similar. There may be some variation and there's a lot of new recipes and new tweaks on these. But this profile is what is stored inside the Impedance Track gauge and can be changed depending on your battery so that you can use the open circuit voltage in the appropriate times to predict the state of charge under any given temperature.
Note also that the open circuit voltage is a function of temperature as well. When it gets colder, the potential inside the cell will increase, so the voltage will be a little bit higher at low temperatures. And at high temperatures, the voltage will be a little bit lower. So you not only need one static table of voltage at a particular temperature, you need to know how that changes at other temperatures as well.
So as long as your cell matches a particular profile, you're going to have very small voltage deviation and your average error can be pretty small. And you can use that same chem ID for a number of different batteries, or you might need to change to use a different chem ID if you're in a different family of voltage curves depending on your gauge.
When we discussed voltage-based gauging, we talked about the challenge of the transient response of the battery, how when you remove a load, the voltage doesn't immediately bounce back to the steady state. It takes some time for the chemistry to get back into a relaxed condition. And that's the time when you need to measure, so Impedance Track algorithm handles that very well and it knows how long it needs to wait, when to take the measurement, and when it's stabilized.
This also gives you the benefit of the voltage-based method that self-discharge estimation is eliminated. You're not estimating, which is inherently going to have some error. You can directly measure the voltage and know the SOC as it decreases over time when your battery is sitting on the shelf.
Now, here's the key point about Impedance Track. It's how the impedance is measured. So I mentioned inside the gauge, we have a fixed table that's the OCV depending on state of charge and temperature. And then the IT algorithm is doing the real time measurements of the voltage when it's under load. And it's doing coulomb counting to track the state of charge.
So we saw this equation before. The voltage is going to be your OCV minus your IR drop. So the gauge can measure the loaded voltage, which is here on the left side of the equation. And it's measuring the current and doing coulomb counting. And so it's tracking its way down this curve, so at any given time it can do the reverse lookup to calculate what the OCV would be.
So that means let's say we start at a full battery. There was no current. It was 4.2 volts. We know we're starting at 100% charge.
Well, then we start discharging and then the voltage is decreasing. And we're counting the coulombs, so we're tracking our way along here. And we know we've counted 25% of the battery capacity, for example. So we know we're at this point right here.
We can back-calculate what the OCV would be at this point and use that in our equation. And if we solve this equation for resistance of the battery, we know OCV by using our table. We know the voltage of the battery under load, because we're measuring it right now. And we know the current at this very moment.
That means we can calculate the resistance of the battery using Ohm's law. So we know the resistance at this particular point in time. And we can continually measure it as we're discharging. And we can store it in our table that is a function of state of charge. And we also know how it varies with temperature as well.
Another thing that Impedance Track gives you is that you don't have to learn the battery capacity by doing a full discharge and a full charge. For this example, we'll talk about how batteries used to be calibrated before they were shipped when they had a fuel gauge inside the battery. Normally you'd have to fully discharge it to empty and then charge it back up to full so that the gauge could learn what is a full battery capacity. And then all batteries, all lithium ion batteries are shipped that about 50% capacity for safety and for durability. It reduces the aging if you store it at 50%.
So that's obviously a very long procedure to do in production. And so most battery makers like to skip that if they have a fuel gauge inside. But even in the case where this is in use by your end user, you don't want them to have to go fully to empty periodically to let the gauge learn the capacity as it fades over time. So let's look at how Impedance Track can overcome this problem.
Qmax we described in the previous sections is the full battery chemical capacity and everything is relative to that. So how can you learn as it fades over time without having to go from full to empty or empty to full? Impedance Track, because it has these highly accurate voltage measurements periodically, can use these as reference points and then integrate the current that passed in between.
So let's use the example where a system was discharged to some point. It was turned off. The battery voltage relaxed and the gauge measured the voltage. And now it knows a very accurate State Of Charge, and we'll call it SOC1 in this equation down here.
Then it's charged for let's say 50% capacity roughly. And then it's turned off again and it gets another voltage measurement. And we'll use that to calculate SOC2. So if we've gone from say 25% to 75%, now we have a 50% delta change and we integrated the current that it took to get between those two points. Therefore, we know that-- let's say if it were 500 milliamps passed charge to get from 25% to 75%, we know that the total capacity should be 1000 milliamp hours. We're just scaling what we saw pass to estimate what the complete total capacity of the battery would be.
In this example, we showed the charging direction. But it works just as well in the discharging direction. If you had a relaxed voltage measurement, discharge, integrate the current, and then get another relaxed voltage measurement. Then you can estimate the Qmax in the same method.
This is a simplified view of what the Impedance Track algorithm is doing and it doesn't have to meet these exact conditions. You can have intermittent discharges and charging, because this particular case may not always be met in the field. But it gives you the idea of how Qmax can be learned without having to do a full discharge.
Let's look at how Impedance Track combines the voltage correlation method and the current integration method in a typical system. On the voltage axis, this is obviously not a single-cell battery. This is several cells in series, but the same principles apply here.
So the pink curve is the battery voltage, and the blue curve is your current. And so you can see that it's charging here and the pink voltage is increasing, even as the current here on the left axis is decreasing and tapering until it shuts off and then relaxes for some time. During that relaxation, the gauge will take the opportunity to get an OCV measurement and update the DOD.
Then the discharge begins in this blue section. And during the discharge, the gauge is coulomb counting to note what DOD or SOC it's at. And then periodically, it's calculating what is the resistance at each point and updating its table that is stored in nonvolatile memory. So it's learning the resistance during this discharge since it's changing over time and it varies from cell to cell. And so it's getting more accurate during the discharge, even as the discharge is occurring.
Then when the discharge stops, the battery voltage relaxes slowly back up to a stable value and the gauge takes another voltage measurement. And if there were any coulomb counting error, it would be corrected by this voltage measurement here. And since we had a voltage measurement before the discharge and after the discharge and we counted the coulombs between these two voltage measurements, we can now update Qmax.
So we've updated our resistance during the discharge. We've updated our Qmax based on the old as measurements on either side of the discharge. And so that's how it combines the voltage correlation with the current integration to continually update state of charge and also to update its predictions and increase the accuracy as it learns the resistance.
So on the Impedance Track fuel gauge slide we're only showing advantages, unlike the previous methods which have disadvantages and advantages. We have the advantages of both voltage-based and coulomb counting methods. That means it's going to work whether you have a small current or no current and you're using open circuit voltage measurements, or whether you have a large load current. So it works across a wide range of loads.
You don't have to use self-discharge modeling. You can use the OCV reading to update as your battery's sitting on the shelf. And it's going to work equally well whether you have a new battery or an aged battery. And batteries will be aging at different rates depending on the conditions they're used in, so you can't model aging. The only way to update it is through Impedance Track by directly measuring the resistance of the battery and periodically updating Qmax.
And to learn the Qmax, you don't need to do a full charge. You don't need a full discharge. It's much more relaxed on how it does its learning.
So if you have an Impedance Track fuel gauge, these are some of the benefits you're going to get. You're going to get remaining run time, which the other methods wouldn't give you, because it's going to do a predictive algorithm. And as it updates resistance or as the temperature changes or as the load changes, it can update the run time predictions.
It's going to give you the ability to do smarter power management in your system, because you can trust what the fuel gauge is saying. You can know when you're getting empty. You can know what state you're at at any given time, and your system can make better decisions.
And as we mentioned before, it's going to give you longer run time, not by giving more energy and not by making your system more efficient, by letting you do smarter power management, letting you trust the accuracy so you can have a lower guard band for your shutdown voltage and also to give you a variable shutdown voltage, depending on the temperature, discharge rate, and the age, all based on its impedance measurements used in the predictions.
And the goal ultimately is to give you an orderly shutdown. You don't want your user to have a sudden shutdown of the system and a nasty surprise. You don't like that when it happens on your PC and you don't want that to happen on your cell phone or any electronic portable product that you're using, so early warning is important to have an orderly shutdown. But at the same time, you don't want to guard band too much and shut down too early and then you're wasting run time.
So let's look at an example of how this accurate gauging can give you more run time. A lot of systems might just use a fixed voltage as their shutdown point. Whether they're using a gauge or not, they may select a very high shutdown voltage unnecessarily, because they don't trust their gauge or because they want to compensate for aged batteries or cold batteries.
And a lot of systems need some extra shutdown capacity that we'll call reserve capacity or reserve energy, so that they can shut down in an orderly fashion, kind of like your notebook PC telling you it's time to shut down, save some data to the disk or to the flash, and then shutting down so you don't lose your work. And a lot of these systems the system designers would select 3.5 volts or 3.6 volts so that they can cover the worst case and make sure they always have an orderly shutdown. So that's what we use for example here.
And then the gauge will be computing remaining capacity and alter the shutdown voltage until there is exactly the reserve capacity left under any particular condition. So we're going to compare using this fixed voltage to using what the gauge predicts as a variable shutdown voltage. And we'll say the system only needs 10 milliamp hours in order to do an orderly shutdown. And in the next slides we'll show you different temperatures and different ages of batteries and compare the two methods.
On the first slide, let's have a new battery and we'll be showing a variable load. So you can see the battery voltage here, there is no load then there's a light load, then a heavy load. Then it relaxes with no load, then a light load, and a heavy load. And it repeats this light load, heavy load, no load until you reach time to shut down your system.
So if we're at room temperature with a new battery and you just have a fixed 3.5 volts as your shutdown point, you might shut down right here and you're going to run for two hours. But if you trust your gauge is going to give you enough warning time to shut down and you know your system can actually shut down at 3 volts, that's where it's going to crash, and you want 10 milliamp hours warning so that you can do that shutdown, Impedance Track gauge is going to give you a warning at about 3.3 volts, which means you're going to get 168 minutes of run time. So by lowering the terminate voltage for a new battery at room temperature, you got a 40% increase in run time using the same battery, the same system. This is just because you can trust the gauge to give you an early warning before your actual final shutdown point.
Let's make it a little trickier and say this is an old battery with a higher internal impedance. That means when you have a load, the voltage is going to drop much more. So during these heavy loads, you're probably going to hit your 3.5 volts much earlier. Remember, for a new battery, it was going into 120 minutes. Now we're only going 90 minutes.
But if you're still using Impedance Track gauge, you know it's going to warn you 10 milliamp hours before you hit 3 volts. You're still going to get 142 minutes run time. It's not a very significant decrease from the new battery. So you didn't have to guard band by using this artificially high voltage. So now we have 58% increase by using the Impedance Track gauge instead of just using a gauge with a fixed shutdown voltage.
Next variable we'll do will be temperature. This is even more dramatic than an old battery. So we're going back to new battery but a cold temperature, zero degrees. Still want the gauge to tell us 10 milliamps before we get 3 volts. And here, we almost got 120 minutes of run time with this cold battery.
But you can see it's much worse to be cold than to be old if you're a battery. And so if you're shutting down at 3.5 volts, you're only going to get 53 minutes of run time. And you might have been surprised as well when you suddenly hit that if this was your load profile. So a very dramatic difference if you allow your gauge to give you early warning and a much lower terminate voltage.
So let's combine these two and go to the extreme case. We have an old battery at zero degrees Celsius using the same load profile. The old battery has a higher internal impedance. The cold battery has a higher internal impedance. You combine those, you can see even on the first heavy discharge you're going to go under your 3.5 volt terminate voltage and get 21 minutes of run time, compared to when it was new and room temperature and this method gave you 120 minutes of run time.
Using Impedance Track, going all the way down to 3 volts with a 10 milliamp hour early warning time, it's going to give you a warning at 3.061 volts. You know you still have 10 milliamp hours in order to complete your shutdown tasks, so your total run time is 82 minutes. It's not as good as a new or a room temperature battery, but it's not bad. It's much closer to what a new room temp battery was. And it's definitely almost 3x what you would get if you just had a fixed shutdown voltage with this load profile.
So that's what we mean when we say Impedance Track and accurate fuel gauging can give you a longer run time for your system with the same battery. And we'll review some of the advantages here before we wrap up this part. It has a dynamic learning ability. It's the only algorithm available that can learn as your battery ages. It also is the only predictive algorithm that can take into account the cell impedance when your temperature changes.
And it has other additional features which we're not covering in this, such as thermal modeling to predict the self-heating of the battery. It has transient modeling to predict the transient voltage response of the battery. And you might have also been thinking, well, I still need in the previous example some guard banding, because I have a very spiky load. We showed a very steady load. We had a light load that was very steady then a heavy load that was very steady. But even if your load has very spiky current and voltage periods, the gauge is going to learn those spikes and also give you an early warning so that those spikes aren't suddenly going to shut your system down by going below your terminate voltage. So again, you can still trust the gauge, even if you have a very spiky load profile on your system.
Aging battery is no problem for Impedance Track. Can adjust the usable capacity by learning the Qmax and the resistance as the cell ages. It's going to increase your run time, because you can use the lower terminate voltage because you can trust the gauge. You can trust it's learning your battery. You can trust that it's learning your system load profile and that it's going to make accurate predictions.
And it gives you the most flexibility. You can update the cell profile in our gauges to match your particular battery and for your system characteristics. So there's a lot of knobs to tweak in the nonvolatile memory of our gauges to make it very flexible to handle any kinds of conditions, any kind of cells, any kind of systems. And it saves your host from needing to try to do all this complicated calculations or gauging algorithms that are inherently going to be inaccurate, but also take processing time and power, and mean your application processor is going to need to be running to do all these algorithms. You can let the low power fuel gauge handle all this and give a warning to the application processor in the system when necessary.
Let's look at a few more considerations on the impact if you leave some energy in your battery because you were using too high of a terminate voltage, for example. Just picking some typical numbers here, for every 100 milliamp hours larger battery that you need, you're going to be paying about $0.15. So if you can lower your terminate voltage, effectively you're getting a larger battery capacity for free. So if you can lower your terminate voltage by 500 millivolts from 3.5 volts to 3 volts, you're going to get about a 5% increase in battery capacity. That's going to save you $0.10 if you're buying a 1,500 milliamp hour battery instead of needing to buy a larger battery.
And for an aged battery, the savings are even more. If you're trying to cover the worst case of an aged battery and you want to buy a larger battery than you really need to handle your high terminate voltage, you might need to spend an extra $1 to get that large enough battery. So this is going to save you money on your new system. It's going to extend the end user run time, without requiring a larger, bulkier, and more expensive battery.
There's also a cost if you have an inaccurate gauge in other ways, not just in your battery capacity. But if your user profile is something like this and is cycling about once a day, after 90 days you might imagine in three months the battery impedance has almost doubled and they have an aged battery. If you're not using Impedance Track-based gauge that is tracking the battery impedance, this means the gauge is becoming less and less accurate every day, so the user is going to get a shorter run time obviously. And if the gauge still making predictions based on the assumption that it's a new battery, then you're going to have a system crash as the battery runs out of juice.
So if your system has a warranty of one or two years and the user was using it in this case and they're getting system crashes, they may return the entire unit because of the faulty gauging results. And obviously, warranty returns are going to cost money to the company. So Impedance Track gauging is going to extend your run time but maintain the accuracy even as the battery ages to prevent the warranty returns and keep the users happy.
This is the end of our Single-Cell Fuel Gauging 101, so we'll just summarize that accurate gauges for your system are just as important as increasing the efficiency of your system or as important as having a larger battery. And in future videos, we'll talk about some of the different gauges that are available from TI and some of the different trade-offs. We talked about pack side versus system side, and there's a bunch of other features and other variables that we'll talk about in future videos. So thanks for sitting through Single-Cell Fuel Gauging 101, and we hope you'll come back for some more videos soon.

Details

Date:
January 1, 2012

Learn about Texas Instruments’ patented Impedance TrackTM technology and the advantages it offers. In addition it will outline the overall benefits of fuel gauging and the consequences of not implementing gauging correctly within your system.